This invention relates to silica-based humidity indicators.
Cobalt chloride indicator gels are used in a range of applications, e.g. to indicate moisture change in gas drying columns. Other drying applications include their use in transformer breathers, tank breathers, in the protection of electronics and telecommunication systems and in laboratory desiccators. It is estimated that approximately 2000 tonnes of cobalt chloride indicator gel are used annually on a global basis.
Cobalt-containing gels for use as humidity indicators have been disclosed in U.S. Pat. Nos. 2,460,071 (disclosing cobalt chloride), 2,460,069 (disclosing cobalt bromide), 2,460,073 (disclosing cobalt iodide), 2,460,074 (disclosing cobalt thiocyanate), 2,460,065 (disclosing cobalt sulphate) and 2,460,070 (disclosing cobalt phosphate).
Indicator silica get is currently produced by impregnating humidified silica-gel or a silica hydrogel with a cobalt chloride solution to produce a dry granular end-product which contains a minimum of 0.5% cobalt chloride and which is blue in colour, changing to pink when water has been adsorbed. Humidified gel is silica gel that has been saturated with water from the vapour phase in order to avoid decrepitation upon impregnation. If the cobalt chloride solution is added directly to the dried gel, the grain size is reduced.
Cobalt chloride has recently been classified as a Category 2 carcinogen (notification from the EEC, Dec. 15, 1998) with the consequence that the use of cobalt chloride indicator gel in industrial applications will require much tighter control to ensure exposure limits are strictly controlled. If acceptable alternatives to the cobalt chloride indicator gel were not available to indicate when saturation had occurred in gas/air drying applications, for instance, this could have serious implications on the users"" downstream processes, e.g. corrosion through moisture damage.
U.S. Pat. Nos. 2,460,072 and 2,460,067 also disclose copper(II) chloride and copper(II) bromide, respectively; but these compounds are not considered suitable candidates for a commercial silica gel-based humidity indicator because of potential toxicity and environmental considerations.
It has been demonstrated that the vanadium compound VOCl3, when impregnated into silica gel gives a colour change from colourless to yellow to orange to red to brown as humidity increasesxe2x80x94see the following references:
Belotserkovskaya et al., xe2x80x9cIndicator properties of vanadium-modified silicas and zeolitesxe2x80x9d Zh. Prikl. Khim. (Leningrad), 63(8), 1674-9;
Malygin, A. A. xe2x80x9cSynthesis and study of physicochemical properties of vanadium-containing silicaxe2x80x94a humidity indicatorxe2x80x9d, Sb. Nauch. Tr. VNII Lyuminoforov I OsoboChist. Veshchestv, 23, 24-8; and
Malygin, A. A. et at, xe2x80x9cStudy of properties of vanadium-containing silica gelxe2x80x9d, Zh. Prikl. Khim. (Leningrad), 52(9), 2094-6.
However, VOCl3 is corrosive, toxic and difficult to prepare and handle.
The present invention therefore addresses the problem of producing an alternative, safe indicator gel to those which are cobalt-based or contain a transition metal salt which is considered to be toxic.
According to one aspect of the present invention there is provided a humidity indicator compound comprising a silica-based carrier containing an iron(II) and/or iron(III) salts or salts as the active indicator.
A second aspect of the present invention resides in the use as a humidity indicator of a compound comprising silica-based carrier containing an iron(II) and/or iron(III) salt or salts as the active indicator.
According to a third aspect of the present invention there is provided a method of monitoring the humidity level within an atmosphere comprising exposing a dried silica-based carrier containing an iron(II) and/or iron(III) salt or salts to said atmosphere and observing colour changes therein.
Typically, humidified silica gel is used as the carrier; however other forms of silica may be used in the production of the silica-based carrier, e.g. silica hydrogel or dry silica gel. The silica-based material may have any of the physical forms normally available. In particular, the form can be irregular granules or approximately spherical beads (often called spherical or beaded silica gel).
The presence of the iron salt imparts a yellow or amber colour to the dry silica-based carrier. When the indicator is exposed to moisture, it then adsorbs water and the colour is observed to fade until it becomes almost colourless when the silica-based carrier is almost saturated with water. This effect has been observed to be a general effect for those iron salts which have been examined.
Once the extent of exposure of the indicator to moisture has resulted in a change of colour from yellow/amber to almost colourless the silica-based carrier may be processed, e.g. by heating to restore its colour, and re-used for humidity monitoring.
The effect referred to above, i.e. colour change from yellow or amber to almost colourless, has been observed in all iron salts examined, e.g. simple iron salts such as ferric sulphate, ferric chloride or ferric nitrate and salts having at least two cations of which one is iron(II) or iron(III), examples of which are ammonium iron(III) sulphate, ammonium iron(II) sulphate and potassium iron(III) sulphate. The effect has been found to be particularly pronounced for the double sulphates or alums.
While not wishing to be bound by theory, the effect is thought to be related to hydrolysis and formation of coloured, polymeric Fe-hydroxy species. In dry silica, such species are thought to be polymerised, and also bound to the silica, to a greater extent than in humidified gel. The higher the degree of polymerisation, and possibly also bonding to the silica, the more intense the colour.
The effect would appear to be related to pH. Those iron salts which exhibit higher pH values when dissolved in water give more intense colours, and more pronounced colour changes, than those which exhibit lower (i.e. more acidic) pH values possibly due to a higher degree of polymerisation of the Fe-hydroxy complexes. Thus ammonium iron(III) sulphate at 10% by weight in water has a pH of 1.7 and produces a silica-based carrier with a deep amber colour, whereas a 10% solution of ferric chloride has a pH of 1.3 and results in a paler yellow shade. The colour of the simple salts can be enhanced by adjusting the pH to higher values comparable to the alums. This may be achieved by the addition of small amounts of sodium hydroxide solution.
Normally an iron(III) salt is employed; however, ferrous counterparts of the ferric salts may also be used since the ferrous ion readily oxidises to the ferric state.
Typically, the silica gel used has a BET surface area in the range of 200 to 1500 m2/g. The pore volume of silica gel may be in the range of 0.2 to 2.0 ml/g, as measured by nitrogen absorption. For example, Sorbsil desiccant gel (Sorbsil is a Trade Mark of Crosfield Limited) typically has a surface area of about 800 m2/g and a pore volume of about 0.4 ml/g. Surface area is determined using standard nitrogen adsorption methods of Brunauer, Emmett and Teller (BET).
The amount of iron present in the silca-based carrier is preferably at least about 0.01 per cent by weight of iron, determined as Fe, relative to the dry weight of the carrier, typically up to about 2.0 percent and usually in the range of about 0.01 percent to about 1.0 percent by weight of the dry weight of the silica-based carrier. The dry weight of a prepared humidity indicator, based on silica gel, according to the invention can be determined by placing a weighed sample (approx. 20 grams) in an oven at 145xc2x0 C. for 16 hours and then weighing the dried material.
According to another aspect of the present invention there is provided a method of producing a humidity indicator comprising soaking silica-based carrier with a solution of an iron(II) and/or iron(III) salt to secure impregnation of the carrier and drying the impregnated carrier.
Typically the indicator gel is prepared by contacting the silica-based carrier with a solution of iron salt containing 1 percent by weight or higher (up to the saturation point) of the iron salt, e.g. by soaking humidified white silica gel in the iron salt solution. Humidified gel is preferred, but the use of dry gel is acceptable. When dry gel is used, the granules decrepitate, so that the product has a smaller particle size than the original product, but, generally, the particle size is still satisfactory for use as a drying agent. In the case of ammonium iron(III) sulphate (herein referred to as iron(III) alum), the solution may range from 1 percent to approximately 50 percent by weight (saturation at 25xc2x0 C.), or higher at higher temperatures. Preferably, the solution contains 10 to 40 percent by weight iron(III) alum at 25xc2x0 C. The use of a high concentration of iron salt helps to reduce the processing time for preparing the indicating silica-based product. The gel is typically soaked in the solution for a period of from 10 minutes to 10 days, preferably 1 to 30 hours, more preferably 2 to 24 hours. The excess solution is drained and the gel dried at 105 to 230xc2x0 C. whereupon it develops its amber colour. An impregnated product dried in this manner will usually have a weight loss after heating at 145xc2x0 C. for 16 hours of less than 10 percent by weight Preferably, the weight loss at 145xc2x0 C. is less than 2 percent by weight.
The invention is illustrated by the following, non-limiting examples.